Positive Electrode Active Substance For Lithium Ion Batteries, Positive Electrode For Lithium Ion Batteries, And Lithium Ion Battery

ABSTRACT

The present invention provides a positive electrode active material for lithium ion battery which has a high capacity and good rate characteristics can be provided. The positive electrode active material for lithium ion battery has a layer structure represented by the compositional formula: Li x (Ni y M 1-y )O z  (wherein M represents at least one type selected from the group consisting of Mn, Co, Al, Mg, Cr, Ti, Fe, Nb, Cu and Zr, x denotes a number from 0.9 to 1.2, y denotes a number from 0.70 to 0.79, and z denotes a number of 1.9 or more), wherein the coordinates of the lattice constant a and compositional ratio (Li/M) are within the region enclosed by three lines given by the equations: y=1.108, y=−37.298x+108.27, and y=75.833x−217.1 on a graph in which the x-axis represents a lattice constant a and the y-axis represents a compositional ratio (Li/M) of Li to M, and the lattice constant c is 14.2 to 14.25.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a positive electrode active materialfor lithium ion battery, a positive electrode for lithium ion battery,and a lithium ion battery.

2. Description of Related Art

A lithium-containing transition metal oxide is generally used as thepositive electrode active material of a lithium ion battery. Specificexamples of the lithium-containing transition metal oxide includelithium cobaltate (LiCoO₂), lithium nickelate (LiNiO₂), and lithiummanganate (LiMn₂O₄). The complexation of these metal oxides is undergoneto improve the characteristics (high capacity, cycle characteristics,preserving characteristics, reduction in internal resistance, and ratecharacteristics) and safety. The characteristics different from thoserequired for lithium ion batteries in mobile telephones and personalcomputers are required for lithium ion batteries used in large-sizedbattery applications such as car applications and road levelingapplications. Particularly, high capacity and rate characteristics areregarded as important.

Various methods have been used to attain high capacity and to improvethe rate characteristics. For example, Patent document 1 discloses alithium battery positive electrode made of a complex oxide representedby the general formula Li_(w)Ni_(x)Co_(y)Al_(z)O₂ (wherein w=0.90 to1.10, x=0.80 to 0.95, y=0.04 to 0.19, z=0.01 to 0.16, and x+y+z=1.0) andalso describes that a lithium battery positive electrode material, whichhas a large discharged capacity, is reduced in the deteriorations ofbattery characteristics caused by repetitive charge/discharge, issuperior in cycle characteristics, is limited in the generation of gascaused by the decomposition of a positive electrode material aftercharged, and improved in preservability/safety, can be provided.

Also, Patent document 2 discloses a complex oxide represented by thegeneral formula A_(w)D_(v)Ni_(x)Al_(y)N_(z)O₂ (wherein A represents atleast one type selected from alkali metals, D represents at least onetype selected from Mg and B, N represents at least one type selectedfrom Si, Ca, Cu, P, In, Sn, Mo, Nb, Y, Bi, and Ga, w, v, x, y, and zrespectively denote a number given by the following formulae:0.05≦w≦1.2, 0.001≦v≦0.2, 0.5≦x≦0.9, 0.1<y≦0.5, and 0.001≦z≦0.2) as apositive electrode active material in a battery which comprises anegative electrode, a positive electrode, and a nonaqueous electrolyteincluding a lithium salt and can be plurally charged/dischargedreversively. Patent document 2 also describes that this oxide enables asecondary battery positive electrode material to excel in all batterycharacteristics such as high capacitization, long life, ratecharacteristics, high-temperature characteristics, and safety.

-   (Patent document 1) Japanese Patent Application Publication No.    10-321224-   (Patent document 2) Japanese Patent Application Publication No.    10-208744

SUMMARY OF INVENTION

However, high capacitization and rate characteristics are importantcharacteristics required for a battery and there is a room forimprovement of a high-quality positive electrode active material forlithium ion battery.

In view of this situation, an object of the present invention is toprovide a positive electrode material for lithium ion battery having ahigh capacity and good rate characteristics.

The inventors have made earnest studies, and as a result, attractedtheir attentions to the relation between the lattice constant a of thepositive electrode active material, compositional ratio of Li to metals(M) other than Li, and battery characteristics, to find that a batteryproduced using the positive electrode active material has goodcharacteristics if the coordinates of a lattice constant a andcompositional ratio (Li/M) are within a predetermined region on a graphin which the x-axis represents the lattice constant a and the y-axisrepresents the compositional ratio (Li/M) of Li to M.

According to a first aspect of the present invention completed based onthe above teachings, there is provided a positive electrode activematerial for lithium ion battery which has a layer structure representedby the compositional formula: Li_(x)(Ni_(y)M_(1-y))O_(z) (wherein Mrepresents at least one type selected from the group consisting of Mn,Co, Al, Mg, Cr, Ti, Fe, Nb, Cu and Zr, x denotes a number from 0.9 to1.2, y denotes a number from 0.70 to 0.79, and z denotes a number of 1.9or more), wherein the coordinates of the lattice constant a andcompositional ratio (Li/M) are within the region enclosed by three linesgiven by the equations: y=1.108, y=−37.298x+108.27, and y=75.833x−217.1on a graph in which the x-axis represents a lattice constant a and they-axis represents a compositional ratio (Li/M) of Li to M, and thelattice constant c is 14.2 to 14.25.

In an embodiment of the positive electrode active material for lithiumion battery according to the present invention, the coordinates of thelattice constant a and compositional ratio (Li/M) are within the regionenclosed by three lines given by the equations: y=5x−13.311,y=49.737x−142.02, and y=−60x+173.56.

In another embodiment of the positive electrode active material forlithium ion battery according to the present invention, M is one metalselected from the group consisting of Mn, Co, and Al.

According to another aspect of the present invention, there is provideda positive electrode for lithium ion battery comprising the positiveelectrode active material according to the present invention.

According to a further aspect of the present invention, there isprovided a lithium ion battery comprising the positive electrode forlithium ion battery according to the present invention.

Advantageous Effect of the Invention

According to the present invention, a positive electrode active materialfor lithium ion battery which has a high capacity and good ratecharacteristics can be provided.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a graph of “lattice constant a”—“compositional ratio of Li/M”according to an example.

DETAILED DESCRIPTION OF EMBODIMENTS (Structure of Positive ElectrodeActive Material for Lithium Ion Battery)

As the material of the positive electrode active material for lithiumion battery according to the present invention, compounds useful as thepositive electrode active material for the positive electrode of usuallithium ion batteries may be widely used. It is particularly preferableto use a lithium-containing transition metal oxide such as lithiumcobaltate (LiCoO₂), lithium nickelate (LiNiO₂), and lithium manganate(LiMn₂O₄). The positive electrode active material for lithium ionbattery which is produced using materials like the above has a layerstructure represented by the compositional formula:Li_(x)(Ni_(y)M_(1-y))O_(z) (wherein M represents at least one typeselected from Mn, Co, Al, Mg, Cr, Ti, Fe, Nb, Cu and Zr, x denotes anumber from 0.9 to 1.2, y denotes a number from 0.70 to 0.79, and zdenotes a number of 1.9 or more).

The ratio of lithium to the total metals in the positive electrodeactive material for lithium ion battery is 0.9 to 1.2. It is because astable crystal structure is scarcely kept when the ratio is less than0.9 whereas high capacity of the battery cannot be secured when theratio exceeds 1.2.

The positive electrode active material for lithium ion battery accordingto the present invention has the characteristics that the coordinates ofthe lattice constant a and compositional ratio (Li/M) are within theregion enclosed by three lines given by the equations: y=1.108,y=−37.298x+108.27, and y=75.833x−217.1 on a graph in which the x-axisrepresents the lattice constant a and the y-axis represents thecompositional ratio (Li/M) of Li to M, and the lattice constant c is14.2 to 14.25. When the lattice constant c is 14.2 to 14.25 and thecoordinates of the lattice constant a and compositional ratio (Li/M) arewithin the above described region, the battery capacity using thepositive electrode active material can be increased and the ratecharacteristics can be excellent.

Also, the coordinates of the lattice constant a and compositional ratio(Li/M) are preferably within a narrower region enclosed by three linesgiven by the equations: y=5x−13.311, y=49.737x−142.02, andy=−60x+173.56, and the lattice constant c is preferably 14.22 to 14.25.

The positive electrode active material for lithium ion battery isconstituted of primary particles, secondary particles formed fromaggregated primary particles, or a mixture of primary particles andsecondary particles. The average particle diameter of these primary andsecondary particles of the positive electrode active material forlithium ion battery is preferably 2 to 8 μm.

When the average particle diameter is less than 2 μm, this makes itdifficult to apply the positive electrode active material to the currentcollector. When the average particle diameter exceeds 8 μm, voids areeasily produced when the active material particles are filled, leadingto less fillability. The average particle diameter is more preferably 3to 6 μm.

(Structure of Positive Electrode for Lithium Ion Battery and Lithium IonBattery Using Positive Electrode)

The positive electrode for lithium ion battery according to anembodiment of the present invention has a structure in which a positiveelectrode mix prepared by blending, for example, a positive electrodeactive material for lithium ion battery which has the aforementionedstructure, a conductive adjuvant, and a binder is applied to one or bothsurfaces of a current collector made of an aluminum foil or the like.Also, a lithium ion battery according to the embodiment of the presentinvention is provided with the positive electrode for lithium ionbattery having such a structure.

(Method for Producing Positive Electrode Active Material for Lithium IonBattery)

Next, a method for producing a positive electrode active material forlithium ion battery according to the embodiment of the present inventionwill be explained in detail.

First, a metal salt solution containing an oxidant is prepared. Themetal salt is a sulfate, chloride, nitrate, acetate, or the like and,particularly, a nitrate is preferable. This is because the nitrate canbe calcined as it is, so that a cleaning process can be omitted, even ifthe nitrate is mixed as impurities in the calcination raw material, andthe nitrate functions as an oxidant to promote oxidation of metals inthe calcination raw material. The metal contained in the metal salt isNi and at least one or more types selected from Mn, Co, Al, Mg, Cr, Ti,Fe, Nb, Cu, and Zr. As the nitrate of a metal, for example, nickelnitrate, cobalt nitrate, or manganese nitrate may be used. At this time,the metal salt is prepared such that each metal is contained in adesired molar ratio. The molar ratio of each metal in the positiveelectrode active material is thereby determined.

Next, lithium carbonate is suspended in pure water, and then, a metalsalt solution of the above metal is poured into the mixture to produce alithium salt solution slurry. At this time, lithium-containing carbonatemicroparticles precipitate in the slurry. In this case, a sulfate orchloride is washed with a saturated lithium carbonate solution and thenseparated by filtration when the lithium compound does not react withthe metal salt in the heat-treatment. When, like the case of using anitrate or acetate, the lithium compound reacts as the lithium rawmaterial during heat treatment, these metal salts are not washed andseparated as it is by filtration, followed by drying, thereby enablingthe salt to be used as a calcination precursor.

Next, the separated lithium-containing carbonate is dried to obtain alithium salt composite (precursor of a positive electrode activematerial for lithium ion battery) powder.

Next, a sagger having a predetermined capacity is prepared and thepowder of the precursor of a positive electrode active material forlithium ion battery is filled in the sagger. Next, the sagger filledwith the powder of the precursor of the positive electrode activematerial for lithium ion battery is transferred to a kiln to calcine.The calcination is performed by keeping the sagger with heating for apredetermined time in an oxygen atmosphere. Also, it is desirable thatthe calcination is performed under a pressure of 101 to 202 KPa becausethe quantity of oxygen in the composition is increased. The calcinationtemperature is 700 to 1100° C., and the calcination is carried outpreferably at 700 to 950° C. when y in the above formula satisfies theequation: 0<y≦0.5 and at 850 to 1100° C. when y in the above formulasatisfies the equation: 0.5<y≦0.7. The crystallinity of the positiveelectrode active material is largely caused by the relation between thecomposition and calcination temperature. At this time, there is the casewhere even a small difference in composition affects the crystallinityof the positive electrode active material though depending on the rangeof calcination temperature. When the positive electrode active materialprecursor is made to have a proper compositional ratio and calcined at aproper calcination temperature corresponding to the compositional ratio,the crystallinity of the positive electrode active material is improvedto make a high-performance positive electrode active material. Also, thecrystallinity of the positive electrode active material is affected notonly by the above factor but also by the grain size of the precursor andthe amount of lithium carbonate used as the raw material. When theamount of lithium carbonate is large and a lot of lithium is containedin the positive electrode material precursor, the calcination proceedssmoothly. In this case, the lattice constant c is decreased withincrease in calcination temperature whereas the lattice constant c isincreased with decrease in calcination temperature because thecalcination is insufficient.

After that, the powder is taken out of the sagger and ground to obtain apositive electrode active material powder.

In this case, when a nitrate is used as the metal salt to be poured inthe production of the lithium salt solution slurry, a positive electrodeactive material containing oxygen exceeding that in the compositionalformula is finally produced. Also, when the calcination of the positiveelectrode precursor is performed not under atmospheric pressure butunder a predetermined pressure, a positive electrode active materialcontaining oxygen exceeding that in the compositional formula is finallyproduced. When the positive electrode active material contains oxygenexceeding that in the compositional formula as mentioned above, abattery using the positive electrode active material is improved invarious characteristics.

EXAMPLES

Although examples are provided for facilitating understanding of thepresent invention and its advantage, the present invention is notlimited to the following examples.

Examples 1 to 26

First, lithium carbonate to be charged in an amount as described inTable 1 was suspended in 3.2 liter of pure water, and then, 4.8 liter ofa metal salt solution was added to the mixture. Here, the metal saltsolution was prepared in such a manner that the compositional ratio of ahydrate of a nitrate of each metal was that described in Table 1 and thenumber of moles of all metals was 14.

In this case, the amount of lithium carbonate to be suspended is a valueat which x in the formula Li_(x)(Ni_(y)M_(1-y))O_(z) of a product(positive electrode for lithium ion secondary battery, that is, positiveelectrode active material) accords to that described in Table 1 and iscalculated according to the following equation.

W(g)=73.9×14×(1+0.5X)×A

In the above formula, “A” is a value multiplied in order to subtract, inadvance, the amount of lithium originated from a lithium compound otherthan lithium carbonate left in the raw material after filtration besidesthe amount required for the precipitation reaction. “A” is 0.9 when,like the case of using a nitrate or acetate, the lithium salt reacts asthe calcination raw material, and 1.0 when, like the case of using asulfate or chloride, the lithium salt does not react as the calcinationraw material.

Though lithium-containing carbonate microparticles were precipitated inthe solution by this treatment, this precipitate was separated byfiltration using a filter press.

Subsequently, the precipitate was dried to obtain a lithium-containingcarbonate (precursor of positive electrode active material for lithiumion battery).

Next, a sagger was prepared to fill the lithium-containing carbonatetherein. Next, the sagger was placed in an oxygen ambient furnace underatmospheric pressure and heated to 800 to 940° C. for 4 hr. Then, thesagger was kept at this temperature under heating for 12 to 30 hr andthen, allowed to cool for 3 hr to obtain an oxide. Then, the obtainedoxide was pulverized to obtain a positive electrode active materialpowder for lithium ion battery.

Example 27

In Example 27, the same procedures as in Examples 1 to 26 were carriedout except that each metal of the raw material was altered to thecomposition shown in Table 1, and a chloride was used as the metal saltto precipitate a lithium-containing carbonate, which was then washedwith a saturated lithium carbonate solution, followed by filtration.

Example 28

In Example 28, the same procedures as in Examples 1 to 26 were carriedout except that each metal of the raw material was altered to thecomposition shown in Table 1 and a sulfate was used as the metal salt toprecipitate a lithium-containing carbonate, which was then washed with asaturated lithium carbonate solution, followed by filtration.

Example 29

In Example 29, the same procedures as in Examples 1 to 26 were carriedout except that each metal of the raw material was altered to thecomposition shown in Table 1 and the calcination was performed not underatmospheric pressure but under a pressure of 120 KPa.

Comparative Examples 1 to 15

In Comparative Examples 1 to 15, the same procedures as in Examples 1 to26 were carried out except that each metal in the raw material wasaltered to the composition shown in Table 1.

TABLE 1 calcination Li₂CO₃ compositional ratio (%) of each metal in allmetals except Li temperature (g) Ni Co Mn Ti Cr Fe Cu Al Sn Mg (° C.)Example 1 1396 70 10 20 820 2 1406 70 10 20 820 3 1396 75 12.5 12.5 8504 1396 75 12.5 12.5 850 5 1396 70 15 15 820 6 1396 70 15 15 820 7 139670 15 15 820 8 1396 70 15 15 820 9 1396 75 10 15 800 10 1396 75 10 15820 11 1396 75 10 15 820 12 1406 75 10 15 820 13 1415 75 10 15 830 141406 75 10 10 5 830 15 1396 75 10 10 5 830 16 1406 75 10 10 5 830 171396 75 10 10 5 830 18 1396 75 10 10 5 830 19 1396 75 10 10 5 830 201396 75 10 10 5 830 21 1396 75 10 10 2.5 2.5 830 22 1396 77 10 13 860 231396 77 10 13 860 24 1396 77 10 13 860 25 1396 77 10 13 860 26 1406 7710 13 860 27 1552 75 10 15 860 28 1552 75 10 15 860 29 1396 75 10 15 860Comparative 1 1396 70 10 20 840 Example 2 1406 70 10 20 840 3 1396 75 1015 830 4 1406 75 10 15 850 5 1415 75 10 15 850 6 1396 75 10 15 850 71396 75 10 10 2.5 2.5 850 8 1396 75 10 10 5 850 9 1396 75 10 10 5 850 101396 75 10 10 5 850 11 1396 75 10 10 5 850 12 1396 70 15 15 830 13 139677 10 13 880 14 1396 70 10 20 830 15 1396 75 15 10 800

(Evaluation)

The contents of Li, Ni, Mn, and Co in each positive electrode activematerial were measured by induction coupling plasma atomic emissionspectrometry (ICP-AES) to calculate the compositional ratio (molarratio) of each metal. Also, the crystal structure was confirmed to be alayer structure by X-ray diffraction.

Moreover, each positive electrode material was measured by powder XRDdiffraction to find the lattice constant from the diffraction pattern.Also, among the measured factors, the lattice constant a was made to lieon the x-axis and the compositional ratio (Li/M) of Li to M (all metalsexcluding Li) found from MS analysis was made to lie on the y-axis todraw a graph as shown in FIG. 1.

The positive electrode material, a conductive material, and a binderwere weighed in a ratio of 85:8:7. The positive electrode activematerial and the conductive material were mixed in a solution preparedby dissolving the binder in an organic solvent (N-methylpyrrolidone)into a slurry, which was then applied to the surface of an Al foil andpressed after dried to produce a positive electrode. In succession, a2032-type coin cell for evaluation in which Li was used as the counterelectrode was manufactured and an electrolytic solution prepared bydissolving 1M-LiPF₆ in EC-DMC (1:1) was used. Then, the coin cellimpregnated with the electrolytic solution was used to calculate thebattery capacity ratio of a battery capacity at 1 C to a batterycapacity at 0.2 C to obtain the rate characteristics of the battery.These results are shown in Table 2.

TABLE 2 discharged capacity rate lattice lattice lattice (0.1 C)characteristics constant a constant c volume v Li/M (A · h/g) (%)Example 1 2.8759 14.2220 101.866 1.047 189 89 2 2.8761 14.2235 101.8931.000 185 87 3 2.8755 14.2223 101.841 1.060 189 89 4 2.8753 14.2209101.814 1.084 192 89 5 2.8764 14.2299 102.029 1.052 189 89 6 2.876714.2248 101.875 1.081 188 87 7 2.8753 14.2171 101.787 1.046 198 89 82.8746 14.2170 101.738 1.055 188 88 9 2.8755 14.2199 101.827 1.034 19089 10 2.8753 14.2195 101.808 1.066 189 88 11 2.8738 14.2179 101.6911.086 184 88 12 2.8773 14.2262 101.996 1.091 184 87 13 2.8733 14.2182101.658 1.107 184 87 14 2.8771 14.2336 102.038 1.075 190 89 15 2.875714.2263 101.886 1.099 189 87 16 2.8766 14.2326 101.993 1.101 189 87 172.8761 14.2480 102.072 1.025 185 88 18 2.8755 14.2463 102.014 1.055 19089 19 2.8758 14.2462 102.032 1.036 190 89 20 2.8749 14.2460 101.9701.045 186 88 21 2.8749 14.2436 101.950 1.080 189 88 22 2.8766 14.2326101.983 1.061 195 89 23 2.8765 14.2313 101.979 1.047 190 88 24 2.875714.2263 101.886 1.049 190 89 25 2.8746 14.2220 101.776 1.104 187 88 262.8747 14.2220 101.786 1.097 189 89 27 2.8741 14.2188 101.716 1.050 18084 28 2.8770 14.2244 101.963 1.055 183 83 29 2.8754 14.2295 101.8381.060 191 91 Comparative 1 2.8789 14.2311 102.145 1.121 182 85 Example 22.8751 14.2224 101.845 1.021 183 86 3 2.8733 14.2215 101.680 1.095 18186 4 2.8730 14.2197 101.713 1.11 183 86 5 2.8775 14.2256 102.009 1.092176 84 6 2.8741 14.2209 101.729 1.109 182 85 7 2.8744 14.2222 101.7631.043 182 87 8 2.8761 14.2162 101.840 0.980 185 85 9 2.8770 14.2241101.962 1.109 180 86 10 2.8757 14.2211 101.841 1.003 185 86 11 2.876614.2254 101.963 1.007 178 80 12 2.8734 14.2130 101.623 1.116 183 86 132.8772 14.2206 101.949 1.056 183 86 14 2.8749 14.1988 101.692 1.036 17982 15 2.8773 14.2531 102.051 1.053 176 80

When the resultant data of Table 2 is plotted on a graph of FIG. 1 andlinear lines are drawn so as to enclose the plotted points at which goodcapacity and rate characteristics of the battery are exhibited, it isfound that these points are included within the region enclosed by threelines: (1) y=1.108, (2) y=−37.298x+108.27, and (3) y=75.833x−217.1 inFIG. 1.

Generally, considerable time is required to evaluate batterycharacteristics when a positive electrode active material is used for abattery. However, according to the present invention, thecharacteristics of a battery provided with a positive electrode activematerial having a predetermined lattice constant c can be evaluated onlyby writing the above three lines on a graph in which the x-axisrepresents the lattice constant a and the y-axis represents thecompositional ratio (Li/M) of Li to M to determine whether or not thecoordinates of the lattice constant a and compositional ratio (Li/M) arewithin the region enclosed by the three lines. Therefore, the timerequired for the evaluation of a battery can be shortened, whichimproves the efficiency of battery production and reduces productioncost.

Moreover, it is found that if lines are drawn so as to enclose thosehaving more excellent battery capacity and rate characteristics, theyare included within the region enclosed by three lines: (1) y=5x−13.311,(2) y=49.737x−142.02, and (3) y=−60x+173.56 in FIG. 1.

Also, Examples 1 to 26 and 29 each use a nitrate as the metal salt to bepoured and therefore, positive electrode active materials containingoxygen exceeding that in the compositional formula is finally produced.When comparing Examples 27 and 28 using a chloride and sulfate as themetal salt with those having the same condition other than the metalsalt, the battery characteristics are more improved (for example,comparison of Example 10 with Examples 27 and 28).

Moreover, in Example 29 in which the positive electrode materialprecursor was calcined not under atmospheric pressure but under apredetermined pressure, a positive electrode active material containingoxygen exceeding that in the compositional formula is finally produced.Therefore, when comparing Example 32 with those having the samecondition other than the pressure, Example 32 was more improved inbattery characteristics (for example, comparison of Example 10 withExample 29).

Comparative Examples 1 to 13 are excluded from the region enclosed bythree lines: (1) y=1.108, (2) y=−37.298x+108.27, and (3) y=75.833x−217.1in FIG. 1 and had inferior battery characteristics. Also, ComparativeExamples 14 and 15 had inferior battery characteristics because they hada lattice constant c out of the defined range from 14.2 to 14.25, thoughthey were included within the region enclosed by three lines: (1)y=1.108, (2) y=−37.298x+108.27, and (3) y=75.833x−217.1 in FIG. 1.

1. A positive electrode active material for lithium ion battery whichhas a layer structure represented by the compositional formula:Li_(x)(Ni_(y)M_(1-y))O_(z) (wherein M represents at least one typeselected from the group consisting of Mn, Co, Al, Mg, Cr, Ti, Fe, Nb, Cuand Zr, x denotes a number from 0.9 to 1.2, y denotes a number from 0.70to 0.79, and z denotes a number of 1.9 or more), wherein the coordinatesof the lattice constant a and compositional ratio (Li/M) are within theregion enclosed by three lines given by the equations: y=1.108,y=−37.298x+108.27, and y=75.833x−217.1 on a graph in which the x-axisrepresents a lattice constant a and the y-axis represents acompositional ratio (Li/M) of Li to M, and the lattice constant c is14.2 to 14.25.
 2. The positive electrode active material for a lithiumion battery according to claim 1, wherein the coordinates of the latticeconstant a and compositional ratio (Li/M) are within the region enclosedby three lines given by the equations: y=5x−13.311, y=49.737x−142.02,and y=−60x+173.56.
 3. The positive electrode active material for alithium ion battery according to claim 1, wherein M is one metalselected from the group consisting of Mn, Co, and Al.
 4. A positiveelectrode for lithium ion battery comprising the positive electrodeactive material described in claim
 1. 5. A lithium ion batterycomprising the positive electrode for lithium ion battery described inclaim
 4. 6. A positive electrode for lithium ion battery comprising thepositive electrode active material described in claim
 2. 7. A positiveelectrode for lithium ion battery comprising the positive electrodeactive material described in claim
 3. 8. A lithium ion batterycomprising the positive electrode for lithium ion battery described inclaim
 6. 9. A lithium ion battery comprising the positive electrode forlithium ion battery described in claim 7.